1. Technical Field
The present invention relates to a tunable interference filter which selects light at a desired objective wavelength from incident light to emit the selected light, an optical module including the tunable interference filter, and an electronic device including the optical module.
2. Related Art
In the related art, a tunable interference filter has been known in which multiple interference of light is caused between a pair of reflection films to emit light at a desired wavelength (for example, refer to JP-A-2008-76749 (Patent Document 1)).
An optical filter device (tunable interference filter) disclosed in Patent Document 1 has a first substrate and a second substrate arranged to face each other, an optical reflection film disposed on a surface of the first substrate facing the second substrate, and a mirror layer disposed on a surface of the second substrate facing the first substrate.
In the first substrate, an oxide film is formed along the circumference of the surface of the first substrate, and the optical reflection film is formed on a diaphragm supported by the oxide film. That is, a gap is formed between the first substrate and the diaphragm. An electrode is disposed on each of the surfaces of the first substrate and the diaphragm. When a voltage is applied between the electrodes, the diaphragm deflects to the side of the first substrate due to electrostatic attraction, so that a gap dimension between the optical reflection film and the mirror layer changes. Thus, by controlling the voltage between the electrodes, the tunable interference filter can extract light at a wavelength corresponding to the gap dimension between the mirrors from incident light.
In such a tunable interference filter that is disclosed in Patent Document 1, an electrode is disposed on a diaphragm. As the electrode, a film-like electrode is generally used. When such an electrode is deposited, an internal stress acts in the plane direction (direction along a substrate surface of the diaphragm) of the film. The direction or size of the internal stress is determined by a depositing method, a film material, and the like. When the internal stress acts in a direction toward the central portion of the film, the internal stress is a compressive stress. When the internal stress acts from the central portion of the film of the electrode to the outside, the internal stress is a tensile stress. Here, when a compressive stress acts on the electrode formed on the diaphragm, the diaphragm deflects toward the first substrate. When a tensile stress acts on the electrode formed on the diaphragm, the diaphragm deflects in a direction away from the first substrate.
When the diaphragm deflects due to the internal stress of the electrode in this manner, the optical reflection film also deflects according to the deflection of the diaphragm. Therefore, in an initial state where a driving voltage is not applied between the electrodes, it is sometimes impossible to maintain parallelism between the optical reflection film and the mirror layer, causing a problem that the resolution of the tunable interference filter is reduced.